Around 24,000 litres of diesel has been seized at Cairnryan Port by HMRC officers.
The fuel was discovered after a trailer, which arrived by ferry from Larne on Wednesday morning, was intercepted by the officers.
A search revealed 960 large drums filled with diesel,packed in cardboard boxes then shrink wrapped onto pallets.
The lorry is estimated to have contained about 24,000 litres of fuel, with a potential revenue loss of about £17,000.
An HMRC statement said a man from County Antrim in Northern Ireland had been detained and released after interview.
It said investigations were continuing.